The invention relates to a method for operating a fuel cell system with a recirculation blower arranged in a fuel circuit.
Conventionally, in the gas supply to a fuel cell system, oxidizing agent in the form of fresh air is compressed in a compressor and then cooled in a charge air cooler. In addition, the air flows into a humidifier module, in which it takes up water vapor from a cathode waste gas (flowing out on the cathode side) of the fuel cell or fuel cell stack. The humidified air is then passed into the fuel cell or the fuel cell stack, where it takes part in the electrochemical reaction.
Fuel in the form of hyperstoichiometric hydrogen is supplied to the anode of the fuel cell system, for example via a metering valve. Unreacted hydrogen and nitrogen and water vapor which have passed over to the cathode are compressed back to the pressure of the fresh gas by means of a blower (hereinafter, a recirculation blower) and fed back into the hydrogen feed stream.
Since water may freeze at low external temperatures, upon starting of the recirculation, mechanical functioning of blower components such as for example valves and/or pump devices, which are arranged in the above-described circuit, may be impaired, and possibly damaged. Conventionally such components at risk of frost are heated by additional heating elements in the event of a sub-freezing start.
As an alternative and/or in addition to the heating elements, in the event of an imminent sub-freezing start due to a low external temperature the recirculation blower serves to discharge water, for example in the form of water vapor. In this respect, the recirculation blower has a predetermined direction of rotation. However, as a result of the predetermined direction of rotation, the water may collect during the discharge process thereof, in recesses in the fuel return line for example due to differences in level. In this case, once the recirculation blower has been turned off the water in the fuel return line flows back, is not discharged and may freeze.
German patent document DE 10 2004 056 744 A1 discloses a fluid compressor, which ensures draining off of water via a discharge opening, in the case of water being sucked into a pump chamber and condensed therein. In the pump chamber, a base part is arranged in a lower portion. Two rotors, which are arranged in the pump chamber, are turned, so as to pressurize the water in the pump chamber. The discharge opening, which is arranged in the base part, expels pressurized water out of the pump chamber. The base part defines a guide face, which extends away from the discharge opening. The guide face is inclined downwards in such a way that water moves downwards over the guide face to the expulsion opening as a result of gravity. A disadvantage is that, when such a fluid compressor is used, there is again a risk of water freezing in recesses in the fuel return line.
The use of a recirculation blower in an anode recirculation loop of a fuel cell system is well known from the prior art. Such devices are disclosed for example in Published U.S. Patent Application Nos. 2005/112014 A1 and 2002/004157 A1, and in U.S. Pat. No. 5,753,383.
International Patent Application WO 2007/128007 A discloses an anode recirculation system which can avoid the use of a recirculation blower by a method of controlling the pressure at a volume for off gas in correspondence with the anode outlet and optionally with the anode inlet. The idea is to store the anode off gas until a pressure is reached which is high enough to feed the anode off gas back to the anode outlet or in an alternative embodiment to the anode inlet. If such a pressure is reached a valve for the hydrogen supply is closed and valves to fed back the anode off gas to the anode outlet or inlet is opened. If the pressure falls under a predetermined valve the hydrogen supply valve is opened again.
One object of the invention is therefore to provide a method and a device for operating a fuel cell system with a recirculation blower arranged in the fuel return line, which recirculation blower in particular overcomes the disadvantages indicated in the prior art and is inexpensive to produce.
This and other objects and advantages are achieved by the method according to the invention for operating a fuel cell system with a recirculation blower arranged in a fuel circuit of the fuel cell system, by means of which fuel issuing on the anode side is fed back to the fuel cell system on the inlet side. According to the invention, fuel issuing on the anode side is conveyed in a fuel return line, at least in places, in alternating flow directions, making possible a simple separation of water from the gaseous fuel and thus drying of the fuel. The method is preferably implemented in a control unit for the fuel cell system as a switch-off procedure in preparation for a sub-freezing start. The method is carried out repeatedly at low external temperatures and in particular before a “sub-freezing start”, the fuel cell system being switched off in order to discharge water present in the fuel return line. In this case the discharge of water, which cannot for example overcome level differences and/or recesses (i.e., “dead zones”) in the flow direction returning the fuel, is made possible by changing, in particular reversing, the flow direction of the returned fuel in the fuel return line.
Therefore, the recirculation blower being controllable in such a way that the fuel issuing on the anode side and returned may be conveyed in the fuel return line at least in places in alternating flow directions. In this case, it is ensured in the fuel return line by way of suitable means (for example non-return valve, shut-off valve or separator) that the fuel issuing on the anode side and conveyed in the fuel return line is not returned to the anode outlet.
Preferably, therefore, only one or more individual zones between components in the fuel return line are operated with alternating flow directions. For example, zones between the recirculation blower and a separator connected upstream thereof are flowed through alternately in both flow directions by the returned fuel.
Conveniently the excess water or water vapor present in liquid phase is discharged from the fuel return line by means of a separator arranged in the fuel return line. The separator preferably promotes a separation procedure in both flow directions (bidirectional) of the fuel. By discharging water from the fuel circuit, in particular from the fuel return line, the risk of water freezing for example in recesses in the fuel return line is reliably avoided. In this way, the use of additional heating elements to protect components at risk of frost, such as for example valves and/or pump devices, in the fuel return line at low external temperatures is not necessary. The separator is preferably arranged upstream of the recirculation blower in the fuel return line in the returning flow direction of the fuel.
In one possible embodiment, the direction of rotation of the recirculation blower is changed (in particular, reversed) to reverse the flow direction of the fuel conveyed in the fuel return line. In one possible configuration of the method according to the invention, the direction of rotation of the recirculation blower is changed (reversed) for the discharge of water, by an electronic drive controller and/or via a transmission, such that the passage direction is likewise reversed.
In order to achieve reversal of the flow direction of the fuel in the fuel return line, it is also possible to adjust the vane geometry of a rotor arranged in the recirculation blower, by an electric drive. In this case the direction of rotation of the recirculation blower is preferably maintained.
Alternatively, in the case of a switched-off (in particular stopped) recirculation blower and simultaneous opening of a purge valve arranged in a fuel return line, backwards flow of the gas stream in the fuel return line is achieved, wherein the water is drained off to the outside via the purge valve.
Conveniently, during reversal of the flow direction in the fuel return line, and thus during discharge of the water, an ejector arranged in the fuel circuit of the fuel cell, in which ejector a given proportion of the returned fuel is recirculated, is bypassed by means of a bypass line and preferably put out of operation. In this way, during reversal of the flow direction no further fuel (in particular hydrogen) is fed via the fuel feed line into the fuel circuit, whereby no driving force acts or is able to act on the ejector.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.